Sure...but my main point above is that top tube length is meaningless without having the context of seat tube angle and head tube height. Both of those effect the length of the top tube, but may not actually change the range of "stack and reach" that a particular frame will allow. I can show you two road frames with identical top tube lengths, but that would require dramatically different length stems and/or spacers for the same fit of the rider relative to the bike's "touch points".

Basically, I don't think there can be any "rule of thumb" for basing a TT/Tri top tube length off of a road bike top tube length. There are just too many variables in just frame dimensioning alone, not to mention the variables of road bike position vs. TT/Tri position.

My suggestion to the OP is to figure out what position and bars he wants to use on the TT bike first...THEN choose a frame that will get him there with a reasonable stem and spacers. Doing anything else is just guessing.